The invention relates to a laser rotation rate sensor in which two light beams counterrotate in a polygon equipped with reflectors which depends its corners, a signal being derived in on the rate of rotation from the frequency difference. The device further includes, for lock-in suppression, a reflector which is designed and operated as a magnetooptic element.
It is known that laser rotation rate sensors can be used to measure inertial rotation rates in that the difference in frequency between counterpropagating electromagnetic waves is determined. It is further known that this frequency difference disappears at input rotation rates below a certain threshold value and that thus the rotation rate sensor loses its ability to measure low rotation rates. This phenomenon is called the lock-in effect. To avoid the lock-in effect, various measures have been developed which, in principle, are all based on the fact that a zero frequency split is forced onto the ring laser or--in other words--that its operating point is placed at a point outside the lock-in band.
One of these measures is the use of the magnetooptic Kerr effect. In this case, a nonreciprocal (i.e. direction dependent) phase shift is forced onto the light when it is reflected at the interface between two media of which at least one must be gyrotropic.
Thus, a phase shift difference is generated between the counterpropagating waves of such a rotation rate sensor and this phase shift difference leads to the above-mentioned desired zero frequency split according to the following equation: EQU .DELTA..nu.=(.DELTA..phi./2.pi.(c/L)
where
.DELTA..nu.=the frequency difference; PA1 .DELTA..phi.=the phase shift difference: PA1 c=the speed of light; PA1 L=the length of the rotational path.
A corresponding arangement is known from U.S. Pat. No. 4,225,239. In that patent, a magnetooptic metal mirror is inserted in the beam path in addition to the conventional corner mirrors and the beams impinge on that mirror in a grazing manner.
Such a magnetic mirror, in addition to having a sufficient Kerr effect, must also have a sufficiently high reflection capability to be able to serve as a resonator mirror. Both these requirements prevent the use of purely metal mirrors made of ferromagnetic material which, although having a sufficient Kerr effect, do not have a reflection capability sufficient for the above-mentioned use (typical reflection values lie between 40 and 70%). To overcome this drawback, U.S. Pat. No. 4,225,239 teaches that the reflection capability of the pure metal surface can be increased by applying dielectric coatings. However, this reduces the Kerr effect of such a mirror to a considerable degree since, due to reflection in the dielectric layers, only a fraction of the incident electromagnetic wave reaches the magnetized layer.
The Kerr mirror design disclosed in German Offenlegungsschrift No. DE-OS 2,432,479, which comprises an alternating sequence of quarter-wave-length layers of a dielectric material and of a ferromagnetic material, has been found to be technologically difficult to realize. To overcome this technological difficulties, German Offenlegungsschrift DE-OS No. 2,919,590 and U.S. Pat. No. 4,195,908 teach an arrangement whereby a gyrotropic garnet layer is located in front of a dielectric layer system. However, the construction of such a mirror requires provision of a plate of a nonmagnetized garnet material having a gyromagnetic layer and subsequent dielectric layers applied in a suitable manner to the side of the garnet material facing away from the radiation source. Therefore, in spite of antireflection coatings on the side facing the radiation, reflection losses cannot be avoided, nor can absorption losses be avoided in the garnet material itself.
A further drawback of all previously proposed Kerr mirrors is that, in order to maintain the necessary polarization states of the electromagnetic radiation, special additional measures must be taken in the resonator.
Moreover, the manufacture of dielectric layer systems having high degrees of reflection is more difficult for p polarized light, as it is used, for example, for the magnetooptic Kerr effect with transverse orientation of the magnetic field (magnetic field vector perpendicular to the plane of beam incidence) than, for example, for s polarized light.
The object of the invention is to provide a magnetooptic element which, by utilization of the magnetooptic Kerr effect, generates the largest possible phase shift difference and, as a consequence thereof, the greatest possible frequency split of the counterrotating electromagnetic waves, and which exhibits high reflectivity for the laser radiation employed.